Going catless
 

Hi! I have a 2018 brz with a ft speedfactory catted UEL header and Motiv high flow cat front pipe. I'm running the open flash v2 tablet tune. I've been running this set up for about 2 years. Certain points when driving the car and revving up or deceleration, it seems like it should back fire. Like it would sound natural. I don't like popcorn tunes but I'd like a little sound every once in a while. I was wondering if I gutted my cats and ran the catless tune on openflash, what all would change. For better or worse. It just sounds unnatural with no pop or sound ever. Thanks!

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Pop tunes are trash and people that do them are trash scene queens.

I'm not trying to do a pop tune, just something with a little more natural sound to it. Right now it sounds weird, it's hard to explain without hearing it. Sounds like it's missing something

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Pops and bangs are not natural for a small displacement NA engine.
Pretty sure most of us have heard these before.
Gutting the cats will let more of the Subie "rumble" come out but at the expense of volume and drone. It will also stink.

Fify

Just remember that the experience you may be lacking every once in a while will become an experience that is all the time.

Do it!! Your happiness is more valuable than what other people think

Trust me, you will have a lot fun

Just get a catless header and leave cat in the front pipe. No smell and plenty of sound.

Are you talking "subi rumble?"

https://www.youtube.com/watch?v=256n0ht2Sw0

I spent years working to exorcise pops, bangs, and farts from a variety of carbureted engines. The unnecessary racket carried a stigma of sloppy maintenance and poor workmanship.

Sure WEC and some IMSA cars make awful rackets on overrun, but for those things two things, and two things only, are important - power and (sometimes) reliability. When the WEC/IMSA cars ran at COTA a while back I’d go every year. One year every 911 that came by backfired on every upshift. One loud BANG and that was it. Next year the bang was gone. My guess is whoever was supplying the fuel/ignition maps killed it.

I’m not sure what about the extraneous noise is “natural.” It’s caused by unburned fuel igniting in the exhaust. Why is that “natural?”

A) it is gong to be loud!
B) it is going to smell!

As above I would not gut the cats but get a catless header and front pipe. That way you can go back or just go with the cat in the front pipe.

You will have the smell also

Yeah, vacuum metering fuel is inferior for a lot of reasons. A lot of small turbo race cars like rally cars used to use it intentionally for anti-lag, but that has really fallen out with how efficient new turbos are for quite some time now.

Maybe they are trying to hot blow their ebay mock diffusers ala 2010 era F1 :iono:

https://www.youtube.com/watch?v=BmGgvHflXgc

Fully catless I would occasionally get a burble from the exhaust under certain conditions. Just deleting one isn't going to do it.

I do not recommend fully catless if it's a stret car/daily. Exhaust fumes stink a lot and is annoying.

Not really. I have a cat-less header with a stock front pipe. If I'm not mistaking, the header cat is more focused on cold start emissions as it heats up faster being next to the engine and all. It has far less cells doing the work. Once the car is warmed up, the rear cat is doing the majority of the work.

I only ever smell fumes when idling for a minute or two on cold start. I never smell fumes once I get going and especially once the car is warmed up. I don't do much idling during my drives of course, but the car has had to sit in traffic for at least 10-15 minutes in a single spot during traffic from accidents in the past. Never smell anything.

Have you ever driven behind your car while someone else drives it?

So you are saying the primary cat has less cells than the secondary cat? Could be true, but seems to be counterintuitive.

And somehow I have none running this setup for the last 4 years. YMMV.

It's also illegal as hell but YMMV.

I have not, but the concern here was for the smell personally, not to those around you as far I understood from the comment. But I honestly have a feeling those behind me don't smell anything either. If I don't smell anything after idling for 15-20 mins in the car, it's unlikely anyone else is smelling anything.

Trying to dig this information up, I thought I recall this being discussed in the past. But based on what I described, it doesn't sound counterintuitive to me. The rear cat does the big work once the car is warmed up. The front cat is there to assist emissions during cold start as it warms up the quickest. I'll see if I can find where I heard or read this, it's possible I got it wrong. I'm sure someone on the forum knows the answer to this.

EDIT: To add to this, the rear cat is quite a bit larger in size, likely definitely having more cells. Also, tons of people have confirmed that if you remove the rear cat, you will have a smell compared to removing the front cat.

I just don’t see how if the secondary cat does the majority of the work, why it wouldn’t be called the primary cat? Also why would there be 0 sensors on the secondary cat? Seems like you’d want a measurement post secondary cat if it was what really cleaned things up, instead of just post primary cat.

I’d be curious to see where you read this stuff as I could easily be wrong too. Just want to learn more about this stuff.

Do you have a side by side photo?

There is definitely added smell by deleting the header cat, but no where near as nasty as fully catless. I mostly notice it in colder weather under harder throttle when I crack the window. AFR is richer which makes sense.

Fully catless is near unbearable by comparison in the same exact condition. the entire cabin reeks of fumes, where the single cat I get a bit of a smell but it doesn't linger and goes away immediately

The thing I find difficult to see is the cost / benefit of ditching the cat or cats. How many horsepower does going catless free up compared to the environmental and personal comfort costs? Does going catless enhance drivability? Comfort? Reliability?

I realize everyone’s calculus is different, but for me with a street driven vehicle - particularly one that is already in 100hp/liter territory - no thanks. And for those whose calculus differs, okay, ditch your cats. But if you wind up in trouble with the authorities, I will snicker a bit.

I can make a case for a catless header on a street car but not the front pipe. Just deleting the header cat is not noticeable 90+% of the time. I only really notice it during the winter and on cold starts in the garage.

Having gone back and forth from stock to a UEL header and tune, back to stock then onto my current JDL made me notice how I changed my shifting habits with the header/tune. Definitely some drivability perks.

Header sort of unlocks 4000-6000rpm range as usable shift points around town under moderate/harder acceleration as the engine doesn't feel dead when you hit the next gear at 3000-4500rpm(the "dip"). Not something I noticed until going back to stock again and realizing how much I shift at those RPMs every day.

Engine still makes all of its power above 5000rpm, but don't feel like I need to rev the nuts out of it all the time to keep the engine happy.

^100% agree with this. I have after market catted headers (HKS GT-Spec), with a custom dyno tune and it is the low down torque where you notice it most. Top end is a little better, but you can become a little more “shift lazy” cruising around town as you have the extra torque down low.

As for going catless headers, I wanted to stay legal.

OP, I wouldn’t go from aftermarket catted to aftermarket catless as the gains would be negligible and you definitely won’t be legal then. Not sure about how it affects the snap, crackle and popping. If you really wanted that, you could probably get it added via a tune.

I do not unfortunately aside from having the exhaust apart myself in the past. Here are some photos I found from a quick search that give a pretty good visual of size difference.

https://www.ft86club.com/forums/showthread.php?t=134526
https://www.ft86club.com/forums/showthread.php?t=63714

The gains are small, but do exist thanks to relieving the extra bit of restriction from having a cat to going cat-less. This has been posted repeatedly from independent sources and tuners.

My look at the whole thing is, if you're getting an aftermarket header with a high-flow cat, you're already illegal because you're tinkering with emissions related parts. So whether you choose to go with a cat-less or high-flow cat header becomes a preference thing. I went with cat-less because from doing enough reading over the years from peoples opinions and thoughts on this, I came to the consensus that there isn't much of a smell if any, and that there was slightly more gains to be had. Since I was sticking to NA, I made my choice. I have no plans to ever touch my front pipe, nor ever go to a high-flow cat in that department. The smell, alone, would turn me down.

I don't think the "primary" or "secondary" names have anything to do with whether one does more work than the other. Purely named for purpose of understanding their location in respect to the whole exhaust. At least that was my guess. Primary comes first, secondary comes second. I do agree with you on the sensor part and don't have a good answer for that. Tried looking the other day for info to backup my statement but had little luck finding anything at all. I'll post if I do.

The more research I do the more it seems you are right and I am wrong. It’s just so surprising that the smaller cat is the one people remove to make the most power. My 2001 Camry has the same setup (header cat + downpipe cat) and it functions in the way you described. Safe to assume the brz must function the same?

Still unsure haha. I did a bit of reading last night myself and could not find a reasonable verdict. Everyone has mixed thoughts on this. My thought on the primary cat removed making the most power is that it's slowing the initial flow down of exhaust gases unlike the secondary which is way down the line. But I honestly don't know much about any of that other than the fact that the flow of exhaust gases is complicated - hence why we can't just throw on a bigger exhaust or remove a cat and get instant gains necessarily.

In any case, the reason we got into this topic was because of exhaust smell. And I was saying that I don't get any smell once warmed up. I was actually thinking I'll experiment for fun when I get home one day. I'll idle the car for a bit, get out, and see if I smell anything and how bad it is. Because the truth is, I hate exhaust smell and I hate to put others through that as well. That is why I was very careful about choosing my exhaust and took a long time to decide. I was looking for best gains through the power band, little to no smell, OEM+ like sound quality and quietness. I've never had an issue passing emissions testing with my setup either for the past 3 years. I live in southeast PA, so YMMV.

Get a friend or family member to drive your car while you drive behind it in a different car with your window down. Only way to really experience/verify if it stinks or not.

Certainly could, but performing the simple test I described would give me most of the story. If it has little to no smell at all, that gives me comfort and anyone driving behind me should be fine as well. Granted when I'm giving it full throttle, there's probably some smell, but the reality of it is, the stock car smells at full throttle at the tailpipes as well.

I never said anything about full throttle pulls, but rather I meant city cruising (25-45) speeds.

Politely disagree that attempting to check for smell at idle will give you "most of the story."

.

I think any exhaust mod is where I live. Luckily no one is checking.

Smelled many WRX/STI car that are catless .

I don’t think it’s worth it, you need at least one cat.

even if it s smaller, the primary cat is a lot closer to the exhaust valves and that surely affect performance more.. it s the first big obstacle the gases encounter so it s quite big of a restriction

the secondary o2 sensor is placed after just 1 cat because it also aids in correcting the afr via fuel trims during cruising, and the first cat is already more than enough for the o2 sensor to measure different from the first, so that the ECU thinks the cat is working
Also placing the o2 sensor after the second cat would bring it much further from the exhaust valve and also close to ambient air which then makes it useless to perform afr adjustment
the 2 sensors in oem tune try their best to keep afr at precisely around 14.7 during cruising because that s the afr that produces less pollution when coupled with catalyzers

When i tune this car, i disable completely the secondary o2 to the point that you can rip it off and the ECU doesnt care, and offset the primary lambda to get a real cruising afr of 15.5 or leaner, a thing impossible to obtain when on oem tune